Wave Number-1 Research Articles

Wavenumber‐4 Patterns of the Sporadic E Over the Middle‐ and Low‐Latitudes

AbstractThe COSMIC radio occultation observations are used to investigate the wavenumber‐4 (WN4) structure of the Sporadic E (Es) layer during 2007–2018. The WN4 pattern is observed in the Es occurrence rate over the middle‐ and low‐latitudes. This pattern is stronger at the low‐latitudes than at the middle‐latitudes. It shows the peak intensity in latitude bands between 5° and 15° dip latitudes in both hemispheres. The WN4 structure occurs predominantly during daytime. It appears above 110 km in the morning, then descends slowly down to about 105 km, and finally disappears after sunset. The WN4 structure moves eastward with a speed of approximately 90°/day. It is prominent in boreal autumn and summer, and disappears in boreal winter. The dominant role of the DE3 tide for the formation of the WN4 structure is supported by similar features between the Es occurrence rate and the DE3 tide of the zonal wind shear, such as the phase, eastward speed, seasonal variation, symmetrical distribution, and strong activity at the low‐latitudes.

Dynamical Coupling Between the Low‐Latitude Lower Thermosphere and Ionosphere via the Nonmigrating Diurnal Tide as Revealed by Concurrent Satellite Observations and Numerical Modeling

AbstractThe diurnal eastward‐propagating tide with zonal wavenumber 3 (DE3) is an important tidal component due to its ability to effectively couple the ionosphere‐thermosphere with the tropical troposphere. In this work, we present the first results of a prominent zonal wavenumber‐4 (WN4) structure in the low‐latitude ionosphere observed by the Scintillation Observations and Response of The Ionosphere to Electrodynamics (SORTIE) CubeSat mission during May 27–June 5, 2020. Least squares analyses of concurrent in situ ion number density measurements from the SORTIE and the Ionospheric Connection Explorer satellites near 420 and 590 km show this pronounced WN4 to be driven by DE3. Thermosphere Ionosphere Mesosphere Energetics Dynamics Sounding of the Atmosphere using Broad band Emission Radiometry temperatures and Specified‐Dynamics Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension output demonstrate that the ionospheric WN4 structure is driven by DE3 propagating from the lower thermosphere.

Multi-model assessment of the late-winter stratospheric response to El Ni\xf1o and La Ni\xf1a

The impact of El Niño-Southern Oscillation (ENSO) on the late-winter extra-tropical stratosphere (January–March) is assessed in a multi-model framework. Three state-of-the-art atmospheric models are run with prescribed SST anomalies representative of a strong ENSO event, with symmetric patterns for El Niño and La Niña. The well-known temperature perturbation in the lower stratosphere during El Niño is captured by two models, in which the anomalous warming at polar latitudes is accompanied by a positive geopotential height anomaly that extends over the polar cap. In the third model, which shows a lack of temperature anomalies over the pole, the anomalous anticyclone is confined over Canada and does not expand to the polar cap. This anomalous center of action emerges from the large-scale tropospheric Rossby wave train forced by ENSO, and shrinking/stretching around the polar vortex is invoked to link it to the temperature response. No disagreement across models is found in the lower stratosphere for La Niña, whose teleconnection is opposite in sign but weaker. In the middle-upper stratosphere (above 50 hPa) the geopotential height anomalies project on a wavenumber-1 (WN1) pattern for both El Niño and, more weakly, La Niña, and show a westward tilt with height up to the stratopause. It is suggested that this WN1 pattern arises from the high-latitude lower-stratospheric anomalies, and that the ENSO teleconnection to the polar stratosphere can be interpreted in terms of upward propagation of the stationary Rossby wave train and quasi-geostrophic balance, instead of wave breaking.

Climatology of quasi-2-day wave structure and variability at middle latitudes in the northern and southern hemispheres

Climatological structure of the quasi-2-day wave (Q2DW) at middle latitudes in temperature and horizontal winds in the mesosphere and lower thermosphere (MLT) was compared between the northern and southern hemispheres. Determination of the Q2DW in temperature was based on observation data by the Microwave Limb Sounder (MLS) onboard NASA's Earth Observing System (EOS) Aura satellite over 17 years from August 2004 to May 2021 and the Q2DW in horizontal winds was derived from Aura/MLS geopotential height data using balance equations. Amplitudes were maximized in summer in the southern hemisphere and in the meridional wind in the northern hemisphere, but in winter in the zonal wind in the northern hemisphere. Summer amplitudes were larger in the meridional wind than the zonal wind in the southern hemisphere, but zonal amplitudes in winter were larger than meridional amplitudes in summer in the northern hemisphere. Westward propagating zonal wavenumber 3 (W3) was largest in both hemispheres, but in addition to well-known W4, W3, W2 and eastward propagating zonal wavenumber 2 (E2), we also found W1, zonally symmetric standing (S0), and E1. Eliassen-Palm fluxes were derived for each mode. W3, W2, W1, and E2 fluxes were exhibited upward and poleward in January in the southern hemisphere while only W3 fluxes were exhibited clearly upward and poleward in July in the northern hemisphere. The balance winds and radar winds agreed in both amplitude and phase in the southern hemisphere and at lower latitudes in the northern hemisphere in January, and at lower latitudes in both hemispheres in July. Furthermore, the Q2DW is modulation in amplitude and phase from the W3 by accumulating other modes.

Relationship Between Wavenumber 4 Pattern of Sporadic E Layer Intensity and Eastward Propagating Diurnal Tide With Zonal Wavenumber 3 in Low Latitude Region

AbstractIonospheric sporadic E layers are irregularities with enhanced plasma density which occur in the ionosphere E region. In the present work, we have focused on an interesting wavenumber 4(WN4) pattern of sporadic E layer based on the Constellation Observing System for Meteorology, Ionosphere and Climate radio occultation S4 indices observation. It is found that amplitude of WN4 pattern has a significant seasonal dependency with the maximum appearing in summer/autumn and minimum in winter. The seasonal variation of the WN4 pattern is basically consistent with the eastward propagating diurnal tide with zonal wavenumber 3 (DE3) tide features. Inconsistencies, mainly an anomalous weak amplitude during the summer in southern hemisphere and propagation direction (speed), are also found. By removing the zonal ion drift factor effect, WN4 pattern of sporadic E layers exhibits a closer relation with the DE3 tide with larger amplitude in the northern hemisphere and ∼90° shift in longitude on average during autumn in the southern hemisphere. In general, it is reasonable to conclude that DE3 tide greatly contributes to this WN4 pattern of sporadic E layers.

Global characteristics of the westward-propagating quasi-16-day wave with zonal wavenumber 1 and the connection with the 2012/2013 SSW revealed by ERA-Interim

We analyze global characteristics of the westward-propagating quasi-16-day wave (Q16DW) with zonal wavenumber 1 (W1) in the troposphere and stratosphere using zonal wind, meridional wind, vertical velocity, temperature, geopotential, and potential vorticity data from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis during one year from December 2012 to November 2013. The amplitudes of the W1 Q16DW are larger in the stratosphere than in the troposphere, and remarkable amplitudes are found at middle and high latitudes in the Northern Hemisphere (NH). More detailed analyses on the temporal variation in the W1 Q16DW show that this wave is significantly enhanced during the 2012/2013 Stratospheric Sudden Warming (SSW) event, and the strong wave most likely provides additional forcing on the splitting of the displaced polar vortex. Analysis of the Eliassen–Palm flux (EP flux) and its divergence of the interaction between W1 Q16DW and quasi-stationary planetary waves with wavenumber 1 during the 2012/2013 SSW event reveals that it causes an upward heat flux and exerts a westward acceleration on the background winds, indicating that this interaction plays an important role in the eastward stratospheric jet reversal. Moreover, the wave is amplified in the occurrence region of barotropic and/or baroclinic instability, suggesting a local source of the growing W1 Q16DW during this SSW event.

Investigation of dominant traveling 10-day wave components using long-term MERRA-2 database

The eastward- and westward-traveling 10-day waves with zonal wavenumbers up to 6 from surface to the middle mesosphere during the recent 12 years from 2007 to 2018 are deduced from MERRA-2 data. On the basis of climatology study, the westward-propagating wave with zonal wave number 1 (W1) and eastward-propagating waves with zonal wave numbers 1 (E1) and 2 (E2) are identified as the dominant traveling ones. They are all active at mid- and high-latitudes above the troposphere and display notable month-to-month variations. The W1 and E2 waves are strong in the NH from December to March and in the SH from June to October, respectively, while the E1 wave is active in the SH from August to October and also in the NH from December to February. Further case study on E1 and E2 waves shows that their latitude–altitude structures are dependent on the transmission condition of the background atmosphere. The presence of these two waves in the stratosphere and mesosphere might have originated from the downward-propagating wave excited in the mesosphere by the mean flow instability, the upward-propagating wave from the troposphere, and/or in situ excited wave in the stratosphere. The two eastward waves can exert strong zonal forcing on the mean flow in the stratosphere and mesosphere in specific periods. Compared with E2 wave, the dramatic forcing from the E1 waves is located in the poleward regions.

Global wave number-4 pattern in the southern subtropical sea surface temperature

Exploratory analysis using empirical orthogonal function revealed the presence of a stationary zonal wavenumber-4 (W4) pattern in the sea surface temperature (SST) anomaly in the southern subtropics (20°S–55°S). The signal over the Southern subtropics is seasonally phase-locked to the austral summer and persists up to mid-autumn. Thermodynamic coupling of atmosphere and the upper ocean helps in generating the W4 pattern, which later terminates due to the breaking of that coupled feedback. It is found that the presence of anomalous SST due to W4 mode in the surrounding of Australia affects the rainfall over the continent by modulating the local atmospheric circulation. During positive phase of W4 event, the presence of cold SST anomaly over the south-eastern and -western side of Australia creates an anomalous divergence circulation. This favours the moisture transport towards south-eastern Australia, resulting in more rainfall in February. The scenario reverses in case of a negative W4 event. There is also a difference of one month between the occurrence of positive and negative W4 peaks. This asymmetry seems to be responsible for the weak SST signal to the South of Australia. Correlation analysis suggests that the W4 pattern in SST is independent of other natural variabilities such as Southern Annular Mode, and Indian Ocean Dipole as well as a rather weak relationship with El Niño/Southern Oscillation.

Effects of vertical wind shear and storm motion on tropical cyclone rainfall asymmetries over the North Indian Ocean

Tropical cyclone (TC) rainfall asymmetry is often influenced by vertical wind shear and storm motion. This study examined the effects of environmental vertical wind shear (200-850 hPa) and storm motion on TC rainfall asymmetry over the North Indian Ocean (NIO): the Bay of Bengal (BoB) and the Arabian Sea (AS). Four TC groups were used in this study: Cyclonic Storm (CS), Severe Cyclonic Storm (SCS), Very Severe Cyclonic Storm (VSCS) and Extreme Severe Cyclonic Storm (ESCS). The Fourier coefficients for wave number-1 was used to analyze the structure of TC rainfall asymmetry. Results show that the maximum TC rainfall asymmetry was predominantly in the downshear left quadrant in the BoB, while it placed to downshear right quadrant in the AS, likely due to the different primary circulation strength of the TC vortex. For the most intense cyclone (ESCS), the maximum TC rainfall asymmetry was in the upshear left quadrant in the BoB, whereas it was downshear right quadrant in the AS. It is evident for both basins that the magnitude of TC rainfall asymmetry declined (increased) with TC intensity (shear strength). This study also examined the collective effects of vertical wind shear and storm motion on TC rainfall asymmetry. Here, the analysis in case of the strong shear environment (>7 m s-1) omitted for the AS because the maximum value for this basin was about 7 m s-1. The result showed that the downshear left quadrant was dominant in the BoB for the maximum TC rainfall asymmetry. In a weak shear environment (<5 m s-1), on the other hand, downshear right quadrant is evident for the maximum TC rainfall asymmetry in the BoB, while it placed dominantly downshear left quadrant in the AS. In the case of motion-relative wavenumber-1, the maximum TC rainfall asymmetry was dominantly downshear for both basins.

Tropospheric Forcing of the 2019 Antarctic Sudden Stratospheric Warming

AbstractThe strongest and most persistent upward propagation of zonal wavenumber 1 (WN1) Rossby waves from the troposphere on record led to the rare Antarctic sudden stratospheric warming (SSW) in September 2019. The dynamical contribution from instantaneous anomalous WN1 and its linear interference with the climatological WN1 contributed equally to the event. The unprecedented WN1 planetary wave behavior is further attributed to a long‐lived midlatitude circumpolar Rossby wave train in the troposphere that was sustained by anomalous convection, first over the subtropical Pacific Ocean east of Australia and then over the eastern South Pacific. Besides the tropospheric wave forcing, the phase of the quasi‐biennial oscillation in the upper stratosphere also facilitated the weakening of polar vortex. Moreover, this SSW strongly influenced the tropospheric circulation via the Southern annular mode, favoring conditions linked to the 2019 bushfires in eastern Australia.

Study of a Quasi 4‐Day Oscillation During the 2018/2019 SSW Over Mohe, China

AbstractWe present an analysis of planetary‐scale oscillations during sudden stratospheric warming (SSW) events based on data obtained from a meteor radar located at Mohe (MH, 53.5°N, 122.3°E), the Aura satellite and Modern‐Era Retrospective analysis for Research and Applications, Version 2 data (MERRA2). The planetary‐scale oscillations in the mesosphere and lower thermosphere (MLT) region during eight SSW events from 2012 to 2019 have been statistically investigated. Our analysis reveals that the enhancement or the generation of westward propagating quasi 16‐day oscillation with wavenumber 1 (W1) is a common feature during SSWs over MH. A strong enhancement of the quasi 4‐day oscillation during the 2018/2019 SSW is captured by both radar and satellite observations. The amplified quasi 4‐day oscillation has a period of ~4.3 days in both meridional and zonal winds and with a wavenumber of W2 in the zonal component. Using the meteor radar and MERRA2 data, the vertical structure of the quasi 4‐day oscillation from the stratosphere to the lower thermosphere is derived. The upward propagating feature of the quasi 4‐day oscillation in the meridional component indicates that the oscillation is very likely generated in the lower mesosphere. The mesospheric zonal wind reversal after an elevated stratopause event is observed during the SSW, which results in a negative meridional gradient of the quasi‐geostrophic potential vorticity. Our results not only reveal that the amplified quasi 4‐day oscillation in the MLT region is associated with the 2018/2019 SSW but also suggest that the amplification is originally generated around 60 km due to barotropic/baroclinic instability and propagates upward to MLT region.

Investigation of the Occurrence of Nighttime Topside Ionospheric Irregularities in Low-Latitude and Equatorial Regions Using CYGNSS Satellites.

By using multi-satellite observations of the L1 signal-to-noise ratio (SNR) from the Cyclone Global Navigation Satellite System (CYGNSS) taken in 2017, we present the occurrence of nighttime topside ionospheric irregularities in low-latitude and equatorial regions. The most significant finding of this study is the existence of longitudinal structures with a wavenumber 4 pattern in the topside irregularities. This suggests that lower atmospheric waves, especially a daytime diurnal eastward-propagating zonal wave number-3 nonmigrating tide (DE3), might play an important role in the generation of topside plasma bubbles during the low solar minimum. Observations of scintillation events indicate that the maximum occurrence of nighttime topside ionospheric irregularities occurs on the magnetic equator during the equinoxes. The current work, which could be regarded as an important update of the previous investigations, would be readily for the further global analysis of the topside ionospheric irregularities.

Structure, Variability, and Mean‐Flow Interactions of the January 2015 Quasi‐2‐Day Wave at Middle and High Southern Latitudes

AbstractThe structure, variability, and mean‐flow interactions of the quasi‐2‐day wave (Q2DW) in the mesosphere and lower thermosphere during January 2015 were studied employing meteor and medium‐frequency radar winds at eight sites from 23°S to 76°S and Microwave Limb Sounder (MLS) temperature and geopotential height measurements from 30°S to 80°S. The event had a duration of ~20–25 days, dominant periods of ~44–52 hr, temperature amplitudes as large as ~16 K, and zonal and meridional wind amplitudes as high as ~40 and 80 m/s, respectively, at middle and lower latitudes. MLS measurements enabled definition of balance winds that agreed well with radar wind amplitudes and phases at middle latitudes where amplitudes were large and quantification of the various Q2DW modes contributing to the full wave field. The Q2DW event was composed primarily of the westward zonal wavenumber 3 (W3) mode but also had measurable amplitudes in other westward modes W1, W2, and W4; eastward modes E1 and E2; and stationary mode S0. Of the secondary modes, W1, W2, and E2 had the larger amplitudes. Inferred MLS balance winds enabled estimates of the Eliassen‐Palm fluxes for each mode, and cumulative zonal accelerations that were found to be in reasonable agreement with radar estimates from ~35°S to 70°S at the lower altitudes at which radar winds were available.

Track Deflection of Typhoon Nesat (2017) as Realized by Multiresolution Simulations of a Global Model

Abstract Typhoon Nesat (2017) headed west-northwestward toward Taiwan but took a relatively larger northward deflection about 300 km away and then a leftward deflection after landfall at northern Taiwan. A global model MPAS, employing a multiresolution of 60–15–3 km mesh, is used to investigate the underlying mechanisms of the track changes. The global model simulations are capable of resolving the detailed topographical effects of the Central Mountain Range (CMR) in Taiwan, giving reasonable 5 day tracks in agreement with the observations for Typhoons Soudelor (2015) and Megi (2016), and comparing better with the observed deflection of Nesat (2017) than the regional model simulation of WRF. Sensitivity experiments indicate that flattening the CMR only partially reduces the track deflection of Nesat, while the elimination of the initial cyclone over the South China Sea disables the possible Fujiwhara effect and leads to a southward-biased track with much weaker northward deflection. The northward deflection of Nesat is mainly in response to the wavenumber-1 (WN-1) horizontal PV advection as the southerly flow east of the typhoon center is enhanced by convergence with the outer cyclonic typhoon flow and the large-scale southwesterlies. Upward motions and PV in the troposphere thus are much stronger to the east of the center than to the west, resulting in westward translation induced by negative WN-1 vertical PV advection but eastward translation induced by positive WN-1 vertical differential latent heating to the east. Near landfall, with stronger upward motions produced over the northern CMR, vertical differential latent heating averaged in 3–8-km height becomes negative and thus retards the westward translation.

A Statistical Approach to Quantify Atmospheric Contributions to the ITEC WN4 Structure Over Low Latitudes

AbstractBased on multiyear observations of ionospheric total electron content (TEC) and atmospheric wind, we develop a new method to interpret the atmospheric contribution to the ionospheric wave number‐4 (WN4) structure according to their coherences in annual variations. First, the ionospheric WN4 is extracted from the equatorial integrated TEC (ITEC) using the global ionospheric maps. The wind observation from the TIMED Doppler Interferometer on board the Thermosphere Ionosphere Mesosphere Energetic and Dynamics (TIMED) satellite are used to deduce the latitudinal symmetric and antisymmetric components of the zonal and meridional DE3 (diurnal, eastward, and zonal wave number 3), SE2 (semidiurnal, eastward, and zonal wave number 2), and SPW4 (stationary planetary wave with zonal wave number 4). We then develop a regression model to estimate the coupling efficiency, the background diurnal influence, and the associated contribution in ITEC WN4. (1) The zonal symmetric DE3 is the most efficient tide in generating ITEC WN4, while the zonal symmetric SPW4 plays a secondary but varied role. (2) The diurnal variation of background ionosphere/thermosphere influence's amplitude is similar to that of the zonal mean ITEC during the day, with a westward phase velocity. (3) The zonal symmetric DE3 resulted WN4 can reach 7% of the zonal daily mean ITEC, while the observed total WN4 value is about 10%. The contributions by the zonal symmetric SE2, SPW4, and meridional symmetric SE2 are comparable with respective maxima ~1.5% of the zonal daily mean ITEC. The present results confirm former suggestions that symmetric zonal DE3 is the primary source for ionospheric WN4, and the contributions due to antisymmetric wind components are relatively small in ITEC WN4.

Meteorological Scale Correlation Relationship of the Ionospheric Longitudinal Structure Wavenumber 4 and Upper Atmospheric Daily DE3 Tide

AbstractIn order to analyze the relationship between atmospheric tides and the ionospheric day‐to‐day variations, this paper mainly presents the meteorological scale (period is shorter than 60 days) correlation relationship between the daily longitudinal structure wavenumber 4 (WN4) in the ionosphere and the daily diurnal eastward propagating with zonal wavenumber 3 (DE3) tide in the upper atmosphere. We deduce the latitudinal integration of total electron content from the global ionospheric maps in the middle‐ to low‐latitude ionosphere and the upper atmospheric temperature from Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics\Sounding of the Atmosphere using Broadband Emission Radiometry observations in the MLT region. Applying new techniques proposed by Li et al. (2015, https://doi.org/10.1002/2015JA021577), we extract the daily WN4 structure and daily DE3 tide, respectively. We found that (1) the meteorological scale cross‐correlation coefficients are symmetric about the equator and dominate at the low latitudes; (2) there is a remarkable valley value around sunrise: the amplitudes in day time are a little larger than those at night; the phases in day time are stable and change slowly at night and dramatically before dawn; (3) they vary gradually with altitudes, and the vertical propagating wavelength is about 35‐km altitude; (4) they present an obvious annual variation that the amplitudes are larger from June solstice to early north autumn, and the phases are stable in these months; and (5) they display an obvious quasi‐biennial oscillation phenomenon, and in June to August of 2008–2009 the amplitudes almost disappear before dawn.

Climatology and Anomaly of the Quasi‐Two‐Day Wave Behaviors During 2003–2018 Austral Summer Periods

AbstractThe Sounding of the Atmosphere using Broadband Emission Radiometry satellite instrument has been measuring the mesospheric global temperature since February 2002, providing the longest continuous data set to statistically study the climatology and anomaly of the quasi‐two‐day waves (QTDWs), including the westward zonal wave number 2 (W2), 3 (W3), and 4 (W4) modes. It is found that the W2 QTDW generally peaks earliest, followed by the W3 mode with the W4 QTDW being amplified lastly. This temporal pattern is always disordered during the sudden stratospheric warming (SSW) periods. The temperature oscillations of the QTDW usually peak at nearly the same period at different altitudes and different latitudes for a certain case but would be different from year to year. The statistical results show that the periods of the W2 QTDWs are more frequently observed to be at 45–48 hr and the periods of the W3 mode are mostly confined between 45 and 52 hr. Nevertheless, the oscillations of the W4 QTDW peak more dispersedly between 41 and 56 hr. The W3 QTDW is usually the strongest among the three modes, whereas it is sometimes equal to or even weaker than W2 and W4 QTDWs, especially during SSW periods. Besides, we found that the vertical distribution of the W3 QTDW splits into a bimodal structure during the SSW periods of 2006 and 2011. And the solar cycle trend of the QTDWs is frequently disturbed by a 2‐ to 5‐year variability, which seems to be a common feature for all the QTDW modes.

Diabatic Heating and Convective Asymmetries During Rapid Intensity Changes of Tropical Cyclones Over North Indian Ocean

ABSTRACT Rapid changes in Tropical Cyclone (TC) structure and intensity are the most difficult aspects of TC forecasting. The intensity and structural changes associated with rapid intensification (RI) and rapid weakening (RW) instances of TCs of North Indian Ocean (NIO) are analysed based on 5 cases each for RI and RW. The vertical profiles of heat (Q1) and moisture (Q2) based on heat and moisture budget equations are analysed to bring out the role of diabatic heating during the RI and RW processes. The associated structural changes are analysed based on the Fourier first order wave number-1 asymmetry in the precipitation patterns associated with RI and RW instances. During RI process, there are 2 predominant Q1 maxima associated with latent heat release near the TC centre, one in the upper troposphere and the other in the lower troposphere. During RW process, there is a single dominant Q1 and Q2 maxima in the mid-troposphere. Regarding the structural changes, there is a pronounced front-back asymmetry in both RI and RW cases. The asymmetry amplitude in the inner core is lower in the case of RI events but, is quite high for RW events. Thus, RW events are characterised by large convective asymmetry.

Two Day Wave Traveling Westward With Wave Number 1 During the Sudden Stratospheric Warming in January 2017

AbstractQuasi‐two day wave propagating westward with wave number 1 (W1) in January 2017 is studied using global temperature observed by Sounding of the Atmosphere using Broadband Emission Radiometry and wind observed by a meteor radar at Fuke, China (19.0°N, 109.8°E). The amplitude of W1 significantly enhances during January 2017, when two stratospheric warming events occur. The temperature perturbation of W1 reaches maximum amplitude of more than 6 K at latitude ±15° around ~84 km and ~95 km. The structure of temperature W1 is symmetric with regard to the equator. The temporal variation of W1 is consistent with the stationary planetary wave with wave number 2 (SPW2), but contrary to the quasi‐two day wave propagating westward with wave number 3 (W3). When SPW2 is large during two sudden stratospheric warming events, energy transfers from W3 to W1. Two bursts of the 2 day wave in meridional wind observed by the meteor radar are just corresponding to the local maxima of W3 and W1, respectively. We conclude that during January 2017, W1 is generated by the nonlinear interaction between SPW2 and W3. SPW2 which is modulated by the quasi‐16 day perturbation in the stratosphere plays a key role in the energy transmission from W3 to W1, and it is responsible for the 16 day variation of W1.

WN4 variability in DMSP ion densities across season, solar cycle, and local time

AbstractNonmigrating tides are a major coupling mechanism between the different regions of the atmosphere and ionosphere. The wave number 4 (WN4) pattern in the ionosphere has been recognized as originating primarily with the diurnal eastward propagating wave‐3 nonmigrating tide (DE3) in the troposphere, and significant effort has been devoted in recent years to understanding how tidal effects manifest in various physical parameters across a very wide range of altitudes. While DE3 and WN4 signatures have been much studied in the mesosphere‐lower thermosphere region, relatively little is known about how WN4 impacts the ionosphere above the F peak. We present an analysis of WN4 in the topside ionosphere as measured by the Defense Meteorological Satellite Program (DMSP) spacecraft, using monthly averages of total ion densities and composition binned by latitude and longitude. We found that WN4 is most strongly present during September equinox. In May–August, ion densities near 180–270 geographic longitude (GLON) are enhanced and the WN4 pattern moves 5–10° north; in November–February, ion densities are reduced in this region and WN4 moves 5–10° south. No solar cycle effects were found in the magnitude of dN/N. The longitude position of the peaks (phase) was observed to vary by about 10° with F10.7, moving eastward with increasing F10.7. The latitude variation was less than 5° and did not show a trend with F10.7. The WN4 pattern changes rapidly near dawn but is very constant throughout the afternoon and evening in terms of dN/N and drifts eastward at about 2° GLON per hour magnetic local time.